This invention relates generally to material resistivity measuring apparatus in which an electrical signal is transmitted through the material to a receiver, and more particularly the invention is directed to such transmitter and receiver apparatus in which the level of transmitter current is communicated to the receiver for use in material resistivity determination.
The resistivity of a material can be determined by passing a current through the material at one pair of electrodes and measuring received voltage at another pair of electrodes. This is a common practice in geophysical measurements in which a transmitter applies an alternating current to the ground through a first electrode structure, and voltage at the transmission frequency is detected by a receiver through a second electrode structure on the ground and spaced from the first electrode structure. The detected current is a measure of soil resistivity, assuming the transmitter power level (voltage, current) is known.
The resistivity of soils varies over a wide range. Moist soil rich in conductive material such as clay and ferrous material will have a lower resistivity and higher current conduction as compared to a dry sandy soil, for example. Small currents may be sufficient for resistivity measurements in dry sandy soils, but higher current will be required for lower resistivity soils.
Thus, the transmitter must be able to operate at different current levels. In capacitively coupled resistivity measurement systems, the current generated by the transmitter is an AC current at some fixed frequency somewhere in the range from a few KHz to several tens of KHz. The transmitter may be producing as much as 1,000 volts at this frequency to achieve the requisite current through the soil. This is required since the coupling impedance from the electrode to the soil must be much greater than resistance of the soil just below the electrode in order for measurements to be accurately determinative of soil resistivity.
Accordingly, in determining soil resistivity the receiver must be aware of the transmitter current level. However, the only external electrical connections on either the transmitter or the receiver are the electrodes which are placed on and capacitively coupled to the ground. Any other electrical wires would capacitively couple to the electrodes and to the ground thus changing the geometry of the measurement.
The present invention allows transmission of power level settings from the transmitter to the receiver using only the electrodes of the transmitter and receiver.